Method of treating rubber



Patented Oct. 12, 1937 I g r 2,095,673

METHOD or TREATING RUBBER Henry R. Minor, Oak Park, Ill., assignor, by

mesne assignments, to Industrial Process Corporation, SaratogaSprings, N. Y., a corporation of New York No Drawing. Application April 6, 1934, Serial No. 719,301

12 Claims. (Cl. 18-50) .uns invention relates to a method of treating ing agents, such as softenin agents which are rubber and more particularly to a method of volatile under the conditions obtaining, may be breaking down rubber and ofincorporating sofsatisfactorily and efliciently dispersed throughtening agents thereinto without the necessity of out the body of the rubber stock by the use of p ged milling .at heavy pressures. carbon dioxide as a carrier and dispersing agent. 5

Heretofore, the general practice in breaking By this means much of the time and power condown or plasticizing crude rubber, and in mixing sumed in milling operations is saved and a rubcompounding ingredients therein has been to mill her having improved qualities and characteristhe rubber between rolls or in a mixer, such as tics isobtained.

the "Banbury mixer or the Gordon plasticizer, Furthermore, carbon dioxide has a preserva- 10 for a considerable period of time until" the rubber tive effect on the 'protein content of the rubber is plasticized, and then to incorporate into the and has the added advantage over steam or other plasticized rubber in the mill or mixer the varigases that it penetrates rubber more quickly and ous pigments, fillers, wetting agents, softening is readily absorbed or adsorbed by the rubber agents, vulcanizing agents, accelerators, antiwithout introducing moisture into the rubber l5 oxidants, and other treating agents. mass.

The milling-in of these compounding ingredi- It is therefore an important object of this inents requires considerable time, and much difiivention to provide a method for treating rubber culty is encountered especially in stocks heavily to cause a break down of the rubber by subjectloaded with pigments, such as carbon black, and ing the raw or crude rubber to the action of cartoughening agents, due 'to scorching, blistering bon dioxide gas at depolymerizing temperaturesand dry-lapping of the stock in the mill and suband under super atmospheric pressures.

sequently in the calendering or sheeting opera- It is a further important object of this inventions. Wetting or softening agents have been tion to provide a method for impregnating raw used along with the pigments to promote the latrubber with softening agents by the. use of car- 25v ters dispersion in the mix, but even with the use bon dioxide gas under pressure and at an eleof such agents, the results have not been entirely vated temperature as a carrier and impregnatsatisfactory. ing agent.

The quantities of carbon or gas black, for ex- A further important object of this invention ample, which may be successfully milled into a is to provide a method for breaking down rubo0 rubber stock are limited because of this tendency ber whereby a considerable saving in time, labor .of the stock to scorch, blister or dry-lap in the and power consumption over the use of mills,

mill and in the subsequent calendering or sheetalone for this purpose may be realized. 3 p a o s. Other and further important objects of this Furthermore, mills and mixers used for this invention will become apparent from the follow- 35 breaking down and mixing of rubber stocks must ing description and appended claims.

be of a heavy duty type and consume much In accordance with the present invention, raw power.v The large consumption of power by mills or crude rubber is subjected to the action of carin the breaking down and mixing of rubber stocks bon dioxide gas under pressures substantially 40 has given rise to suggestions that it might be higher than atmospheric and at elevated temper- 40 possible to break down or plasticize rubber in the atures for a suflicient period of time to soften or presence of inert gases, such as steam, but such depolymerize the rubber. As a result of this processes have, so far as the present inventor is treatment, the stock is effectively broken down or aware, not been commercially successful. The plasticized and may be more easily milled and use of steam, for instance, introduces water into mixed with the compounding ingredients in a 45 the rubber and necessitates an added step'for its comparatively shorter time and with much less removal from the rubber. consumption of power.

I have now discovered that the breaking down Where softening agents are to be incorpoor ,plasticizing of rubber may be satisfactorily, rated into rubber, this may be done more emcarried out in the presence of carbon dioxide ciently by my method by adding the softening 50 un v d temperatures and pressures. agents, whichare volatile under the conditions Carbon .dioxide acts differently from nitrogen, obtaining, to the raw rubber, without any substeam or other inert gas and is not to be constantial mixing, before subjecting the rubber sidered as an equivalent of such inertgases. mass to the carbon dioxide treatment. In this I have further determined that various treatway, the carbon dioxide acts as a carrier and 55 85 330 to 340 F. and a heating period of two to penetrating agent to disperse the softening agents, and the like, throughout the mass of the of carbon dioxide as a carrier results in a rubber stock having superior qualities, greater tensile strength and greater uniformity.

In carrying out my invention, the raw rubber may suitably be pin sheet form or may be cut into pieces of convenient size and enclosed in a vessel from which the air may or may not have been evacuated. The presence of small proportions of air in my method is not objectionable. Carbon dioxide is then introduced into the vessel and is maintained in the vessel at a pressure of between 25 and 100 lbs. per sq. in., and preferably at about 50 lbs. per sq. in. Pressures in excess of 100 lbs. per sq. in. give no especial added advantage.

The-vessel may be heated, as by steam jacketed walls, to a temperature equivalent to that of steam at 100 lbs/sq. in. pressure and over, viz. to temperatures in excess of about 330 F. Temperatures sufficiently high to depolymerize the rubber under normal atmospheric conditions should be used for a period long enough to break down or plasticize the rubber. Actually, under the conditions obtaining, no substantial depolymerization of the" rubber may take place, owing to the carbon dioxide pressure. The pressures, temperatures and times employed may be varied, as will be understood by skilled artisans; but a pressure of 50 lbs per sq. in.,- a temperature of three hours have been used successfully. In general, the temperatures employed in my process are higher than those normally employed in the vulcanization of rubber compounds.

Various wetting and softening agents which are liquid and/or are volatile under the conditions obtaining may be incorporated into the i ployed are liquid, they may be poured overthe surface of the rubber in the proper proportion either before or after therubber has been introduced into the pressure vessel.

While, in general, it is preferable'in connection with the incorporation of wetting and softening agents into the rubber to use the same temperatures, pressures and times as were specified.

in the method of plasticizing the rubber described above, lower temperatures may be'employed so long as they result in the volatilization of the treating agent, but the full plas'ticizing effect of the carbon dioxide may not be realized at temperatures below the depolymerizing temperatureformly dispersedthroughout the rubber mass,

while the carbon dioxide is given off. The attrac tion bf the rubber particles for these treating agents is apparently greater than the attraction of the carbon dioxide for the treating agents. It may be that the treating, agents are actually condensed upon the surfaces of the rubber particles.

Where test specimens have been made up from rubber treated in accordance with my invention and compared with specimens made up in the usual fashion, it has been found that the time heretofore required to incorporate treating ingredients into the rubber stock on the mill has been entirely eliminated. Furthermore, for example, a specimen in which 2% of pine tar was added in the mill in the customary fashion gave a.tensile strength of 4055 lbs. per sq. in. and an elongation of 665%, whereas when the pine tar was incorporated by my method, the tensile strength under identical conditions was 4440 lbs. per sq. in. and the elongation 660%, indicating a substantial improvement in tensile strength without any decrease in elongation. There is thus a very considerable improvement in the product as well as a reduction in the milling time.

Raw rubber, which has been plasticized by my process requires-she milling other than the mere sheeting out of the material, such as-is done anyway on pre-masticated rubber. Important economies are thus effected by the use of my process.

I am aware that numerous details of the process may be varied through a wide range without departing from the principles of this invention, and I, therefore, do not purpose, limiting the patent granted hereon otherwise than necessitated by the prior art.

I claim as my invention:

l. The method of incorporating a softening agent in rubber, which comprises confining raw rubber and a softening agent to be incorporated into the rubber in an atmosphere of. carbon dioxide at an elevated temperature and under a pressure of at least 50 lbs/sq. in. and releasing the pressure to obtain a rubber having said softening agent incorporated substantially uniformly therein.

2. The method of incorporating softening agents into rubber, which comprises subjecting raw'rubber admixed with a softening agent to the penetrating action of carbon dioxide gas at a depolymerizing temperature and under a pressure of 50 lbs/sq. in. and over and releasing the pressure to obtain a treated rubber having said softening agent substantially uniformly dispersed therein.

'3. The -method of incorporating softening agents into rubber, which comprises subjecting raw rubber admixed with a liquid: softening agent to the penetrating action of carbon dioxide gas at a depolymerizing temperature and under a gas pressure of 50 lbs/sq. in. and over and releasing the pressure to obtain a treated rubber having said softening agent substantially uni formly dispersed therein.

4. The method of simultaneously breaking down crude rubber and incorporating a softening agent therein, which comprises subjecting crude rubber and a softening agent in contact there-- with ma confined atmosphere of carbon dioxide to a depolymerizingtemperature and to a pressure of 50 lbs./sq. in. and over for a period of more than an hour and releasing said pressure to obtain a rubber that is broken down and therefore'more readily milled and contains said softening agent substantially uniformly dispersed throughout the body of therubber.

5.111 a method of treatingrubber, breaking down the rubber in the presence of carbon dioxide under elevated temperatures and pressures, said temperatures being not less than 330 F. I

6. In a method of treating rubber, breaking down the rubber in the presence of carbon dioxide under elevated temperatures and pressures, said temperatures being not less than 330 F. and said pressures b'eing'not less than 50 lbs. pressure of the carbon dioxide.

7. A method of treating rubber, comprising the step of breaking down rubber by subjecting the rubber in an atmosphere of carbon dioxide to a temperature of not less than 330 F.

8. The process of treating rubber to break it down comprising the step of placing rubber in a closed vessel; the step of evacuating airat least partially from the vessel, and the step of. introducingcarbon dioxide to at least take the place of the air thus evacuated, and the step of subjecting the rubber while in the presence of carbon dioxide to depolymerizing temperatures, for in stance, 'at not less than 330 F., or above.

9. In a method of treating rubber, placing rubber under a partial vacuum, adding carbon dioxide gas to restore the atmospheric condition in which the rubber is placed to at least atmose pheric pressure, and elevating the.tempe'rature of the rubber and the gas in which the rubber is placed to depolymerizing temperature of at least stantial mixing;

.mately not less than 330 330 F. and working the rubber at such a temperature and under such a pressure.

10. In a method of treating rubber, placing the rubber in an atmosphere of carbon dioxide gas and elevating the temperature thereof to not less than 330 F. and maintaining that temperature for a sufficient period of time to soften or depolymerize the rubber.

11. In a method of treating rubber, adding softening agents to the rubber without any substantial mixing, placing the rubber in an enclosed vessel in the presence of carbon dioxide and depolymerizing the rubber'and utilizing the carbon dioxide to carry the softening agents into the rubber by subjecting the carbon dioxide, softening agents and rubber to a temperature of not less than 330 E, which is the temperature of ,depolymerization of the rubber.

12. In a method of treating raw rubber, adding softening agents to the raw rubber without subplacing the rubber in an enclosed vessel and partially evacuating the air from the vessel and the rubber; restoring the vessel to atmospheric pressure by adding carbon dioxide gas thereto; and elevating the temperature of thecontents of the vessel until the depolymerizing temperature of rubber, of approxi- F., is accomplished. HENRY R. MINOR. 

